ISDN standards organizations, equipment vendors, and operating companies have generally accepted 5.5 km as the target reach for normal 2B+D basic rate ISDN access service. 5.5 km is a reasonable distance accommodation because (a) it is achievable by signal processing methods which are feasible with prevailing VLSI decision feedback equalization, echo cancellation, and line coding methods using only four signal levels without using sequence estimation techniques, (b) it is a good match to subscriber demography in that most (typically, more than 97%) of all subscribers are within 5.5 km of a central office, and (c) it is compatible with past line loading practice in that, generally, loops shorter than 5.5 km are not inductively loaded and, therefore, are upgradable to digital transmission without the need of removing loading coils. Beyond 5.5 km, loading coils are more common, sharply increasing the cost of a digital subscriber loop installation.
While the industry presently plans to offer fixed 2B+D and 23B+D transmission rates, technology is capable of transmission at intermediate transmission rates. Indeed, it can be shown that, in principle, if one has a technology that achieves bandwidth-limited 2B+D transmission over a 5.5 km loop (f3dB approx. 9 KHz), then the same base technology could support approximately 22B+D transmission over a 1 Km loop (f3db around 200 KHz). There are many business subscribers in a city core who are well under 5.5 km from a Central Office and who could benefit from service capacity between the 2B and 23B limits. ISDN technology and methods are particularly well suited to potentially providing a flexible bandwidth because one D channel can manage the signalling for a varying number of B channels. Prior to the onset of ISDN, it would have been impractical to consider this type of service because there was no workable manner of managing and flexibly exploiting the extra capacity which could be achieved for a subscriber on a short loop, as opposed to a long loop, because of the previous non-channelized, in-band signalling protocols. The ISDN access format is, by comparison, well suited to deal flexibly with different numbers of B-channels for each subscriber or to deal with applications where basic access services are provided to a number of customers over one physical wire pair such as in multi-tenant business buildings, apartment buildings, PBX to Central Office trunk groups. This would make it possible for a telephone company to offer an individual subscriber an mB+D service where "m" is as high as the least of either the capacity requested by the subscriber or the physical transmission limit of the specific loop.
The difficulty with offering this service is that, heretofore, it would be necessary to measure every subscriber loop copper pair to determine the highest theoretical service capacity the loop could support, different interface rate cards would be required for each such rate, and records of the different services would have to be maintained. There is a need therefore for an apparatus and an automatic transmission rate adaptation process for easily and quickly determining the highest, safe bi-directional transmission rate for any given loop and automatically entering service at the rate.
Traditionally, in transmission design of any sort, including conventional ISDN, a bit-rate is determined for a given service and then all efforts focus on the technology to transmit that bit rate the greatest possible distance under the worst feasible conditions. Whereas designers of modems and data transmission applications frequently use buffers, queuing and statistical multiplexing concepts and varying effective transmission rates, in the world of engineering of true circuit transmission services, changing transmission capacities or buffering are considered extremely objectionable properties for a transmission system. Throughout the literature and in experience of the classical circuit transmission engineering, the bit rate is the one "given" parameter; it is the main requirement of the design. The classical challenge is to meet the target bit rate over the maximum distance possible. Thus, the suggestion of changing the bit rate adaptively as a means of ensuring the required BER performance or reach would normally not be acceptable.
However, the recent ISDN context presents thousands (i.e. each subscriber's loop) of individual transmission design problems where it is the length of each loop that is fixed and digital formats sharing common D-channel signalling methods can manage an arbitrary amount of bearer channels simultaneously. Notwithstanding this opportunity, the industry has so far focused exclusively on providing selected fixed transmission rates (144 kb/s, 800 kb/s, 1.544 Mb/s) and hopes to achieve the maximum possible reach for each such bit rate. However, because the length of any one loop is fixed, theoretical reach maximization at fixed bit rates is of no benefit to the vast majority of individual applications.
Looking to conventional methods of transmission rate selection, it is known to estimate a potential transmission rate by impulse response characterization. Thus, given the objective of operating at the highest reasonable transmission rate over an unknown transmission path, it would be obvious to one skilled in the art of digital transmission to use impulse response testing to determine the transfer function of an unknown transmission path and then select a transmission rate that is known to be within workable limits for given coding, modulation and receiver circuit implementation. Use of this approach could in principle take two forms: (a) explicit manual impulse response characterization, followed by setting and verifying the transmission rate or (b) automatic pulsing of the path by the transmitter at one end and corresponding numerical sampling of the impulse response of the line by the receiver at the other end to thereby deduce the transfer fucntion as a means to determining the operating transmission rate.
These methods suffer from a number of drawbacks. Method (a) requires manual involvement and is objectionable for this reason. Neither method (a) nor method (b) directly determines the highest bi-directional rate at which the loop will operate because the impulse response is only related in principle to the potential transmission capacity. Factors such as noise, crosstalk, non-linearity are not taken into account by these methods unless they are also measured separately. Even then, the determination of the highest safe operating rate is not based on direct verification of transmission rates by actual transmission but rather it is based on data acquisition and subsequent theory-based calculations of the expected transmission performance. Both methods also require that impulse response data, and/or other measurements taken at each end, be exchanged between ends so that the adopted transmission rate is the lowest of the two rates determined to be feasible for the two independent directions of transmission. Both methods require significant extra circuit components for the impulse-response stimuli and data acquisition process that are instrumentation-quality linear subsystems and are not a normal part of the transceiver for its operating phase.
Some data modem designs provide transmission rate selection and optimizing techniques. However, these achieve only unidirectional rate measurement or adaptation and/or apply only in the context of data transmission applications where data may be buffered and system control information may be substituted therein whenever required. This is not accepted when providing continuous, uninterrupted customer circuit service to an application, such as in an ISDN environment.
One modem design is commonly known as the "autobaud" modem which allows a host computer to deduce the transmission rate received from a data terminal when that terminal logs on. An agreed to character symbol (determined by convention prior to the use of the system, for instance the letter "o") is transmitted by the modem at the terminal which is logging on. To determine the data rate at which the terminal will transmit during the session, the host receiver simply oversamples the received waveform and, knowing that it is the agreed character, can thereby determine the transmission rate of the data bits from the remote terminal.
Another modem design technique is the method of apparent transmission rate maximization through data compression. The word "apparent" is used because the actual physical symbol transmission rate is not altered. Rather, through source encoding means, which extract redundancy from the source messages, such as by means of partial Huffman encoding, the sending modem replaces only a limited subset of commonly repeated patterns in the data with a special symbol so that the number of symbols of transmission for a given amount of source information is reduce. The receiving modem performs the reverse expansion. In these schemes for data transmission, ACK/NACK (acknowledgement) information is also available from the other end so that the transmitting end is aware of dropping throughput should this occur. Given this measure of channel throughput, it possible to set a criterion at which the extra delay for source encoding (e.g. converting uncoded symbols to less redundant transmission symbols based on Huffman tables of symbol frequency) has a net advantage in effective throughput through the impaired channel. In such schemes however, the physical baud rate of transmission is not actually changed.
Another modem method of transmission rate adjustment is applicable to data transmission but not to continuous transmission systems which provide high speed, low BER, constant delay circuit services. In a data transmission application, error detection information is fed back to the transmitter so it knows the success rate of message transmission continually while in operation. In any case where such information is available, it is a simple matter for a modem to adjust by reducing its transmission rate in half. Many commercial modems "fall back" to 4800, 2400 or 1200 b/s and remain there for the duration of a call if they encounter excessively frequent NACK indications.
A recent variation on this technique but which differs from the present invention is employed in the Ven-Tel Pathfinder (trade mark) 18K modem. This modem uses 18 individually modulated subcarriers straddling the voice frequency transmission band. The modem measures the carrier strengths over the actual connections and, on this basis, classifies each carrier as to its goodness for data transmission (i.e. its strength through the actual channel). When speed reductions are necessary, this modem falls-back in 100 b/s increments by disusing the weakest carriers. When conditions improve, carriers that return to the required strength can be re-entered into service with an increase in overall transmission rate.
The Pathfinder modem application is, again, a "data" application in which continuous transmission feedback is provided and wherein it is possible to change the transmission rate while in service. In ISDN applications, the bearer circuit-service cannot be interrupted, even temporarily, while in service because voice traffic is carried. In addition, multi-carrier channelization FDM methods, as used by the Pathfinder modem, are less efficient and more complex in circuitry than baseband methods of transmission for digital subscriber loop application. Since a Digital Signal Processing (DSP) chip is probably used to perform the power measurement and multi-channel filtering, modulation and demodulation functions in the Pathfinder, the pathfinder method is not presently technologically suited to a scheme which transmits adaptively at rates up to 1.544 Mb/s (rather than 18 kb/s) in both directions. The DSP chip will not be viable for such applications at these rates in the forseable future. The Pathfinder modem method requires processing of signal spectra which are no higher than the edge of the voiceband (4 to 6 KHz) and so numerical DSP is feasible for that application.
The Pathfinder modem method does not provide a true circuit service to the application such as is needed for voice communications in both directions (i.e. once a 64 kb/s stream is said to be available to an application on a circuit basis, the control protocol can never thereafter step in and reduce this amount or even buffer the application's bits temporarily to interrupt with its own control bits), it does not (as far as is disclosed) incorporate a guaranteed working power-margin after adaptation, it does not inherently result in operation at the highest common bidirectional transmission rate as is required in ISDN applications, and the Pathfinder adaptation approach does not apply in situations where it is desired to use baseband transmission methods to implement the technology.